Many techniques have been employed to treat surfaces for the purpose of improving the adhesion of various subsequently applied materials. Such techniques include subjecting surfaces to high energy (e.g., corona discharge, electron beam discharge, flame treatment and the like) and chemically modifying surfaces by chemical etching and the like. Such techniques require the use of expensive equipment and may expose the operator to high energy radiation or dangerous chemicals.
Parachlorophenol has been used as a surface treatment. This material provides only minimal improvement in adhesion compared to the present invention and suffers from the major disadvantage that it is hazardous to human health.
Surfaces have also been coated with primers in an effort to enhance the adhesion of subsequently applied coatings. Primers frequently must be cured after application. This adds time and expense to the manufacturing process. Additionally, the substrate often must be treated in some way (e.g., subjected to high energy) in order that the primer will exhibit sufficient adhesion to it. Primers are also frequently compounded to meet the needs of a specific coating or class of such coatings. For these and other reasons, the use of primers has not proven entirely satisfactory.
Yet another approach is to incorporate aziridine-containing materials into functional coatings which contain groups reactive with the aziridine functionality. These coatings are then applied to a substrate. See, for example, U.S. Pat. Nos. 3,017,280; 4,167,414; 4,181,528; and 4,241,169.
These references disclose that the substrate and the functional coatings must each contain groups reactive with the aziridine group so that the aziridine essentially acts as a tie layer to better anchor the functional coatings to the substrate through crosslinking reactions. They also generally disclose that substrates which do not inherently contain the requisite surface functionality to react with the aziridine (i.e., inert materials) must undergo prior modification by oxidative treatment to incorporate that functionality. Such substrates include, for example, conventional polyesters such as poly(ethylene terephthalate), conventional polyolefin films such as polyethylene, polypropylene and polystyrene.
U.S. Pat. No. 4,418,164 discloses coating aramid filaments with polyfunctional aziridines. The coated filaments are used as reinforcing fibers in unsaturated polyester matrices wherein they are totally encased in the matrices.